Of the trialkylacetic acids, pivalic acid [(CH.sub.3).sub.3 CCOOH] is one of the most widely used. It is a fine chemical suitable as a starting material for various agrichemicals, pharmaceuticals, aroma chemicals, specialty chemicals, and polymer additives. Trialkylcarboxylic acids are typically made in large scale commercial amounts using a non-catalytic process involving substantial amounts of highly corrosive liquid acidic reactant, often containing boron trifluoride. The acidic reactants are typically present in molar amounts equal to or greater than that of the branched olefin feed.
Typical of this technology is U.S. Pat. No. 3,068,256 to Roming. This patent describes a process where the material characterized as the catalyst (liquid mineral acids having a specific gravity greater than 1.25, e.g., H.sub.2 SO.sub.4, BF.sub.3 2H.sub.2 O, or mixtures of BF.sub.3 2H.sub.2 O with H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4) is used in amounts approximately stoichiometric with the olefinic feed. However, this catalytic material must be recovered by hydrolysis of an intermediate complex generated by stoichiometric reaction of olefin, carbon monoxide,and the acid. Consequently, the disclosed process requires a large catalyst inventory; it also requires expensive materials of construction suitable for handling that large corrosive acid catalyst inventory. Furthermore, the hydrolysis step is very difficult to control if optimal regeneration of the acid catalyst of its most active state is desired.
In summary, the process disclosed in the Roming patent is not truly catalytic in nature in the sense that large amounts of the acid along with the feed olefin, carbon monoxide, and water are reacted to form the product carboxylic acid. The catalyst separation and recovery steps are complex.
Similarly, U.S. Pat. No. 3,061,621 to Koch et al., discloses a process for the production of carboxylic acids from olefins and carbon monoxide using a molar excess of an acid catalyst selected from orthophosphoric acid, derivatives of orthophosphoric acid, higher polyphosphoric acids, and their mixtures. This process involves use of large amounts of the catalyst which must be regenerated and recycled in a complicated series of steps.
The process shown in U.S. Pat. No. 2,831,877 to Koch et al., involves two separate steps. In the first step, feed olefin and carbon monoxide are contacted with very strong, and potentially very corrosive, acid catalysts. Suitable acids are said to include 90% to 100% sulfuric acid, anhydrous HF, anhydrous HF-BF.sub.3, chlorosulfonic acid, and fluorosulfonic acid. The resulting intermediate formed upon stoichiometric reaction of the acid, olefin, and carbon monoxide is hydrolyzed with water to liberate the product carboxylic acid. The catalyst must be regenerated to its anhydrous form prior to re-use in the process. This step entails treatment of a very corrosive stream containing significant amounts of water. Clearly, costly corrosion-resistant materials of construction are necessary. A preferable process would entail the use of a catalyst having a substantially less corrosive catalyst and a more economical catalyst recovery step.
A further U.S. patent to Koch et al. (U.S. Pat. No. 2,967,873) describes a process for the production of alkyl esters of carboxylic acids. The process involves two steps. This first step involves reaction of an olefin (containing at least six carbon atoms) with carbon monoxide using approximately a stoichiometric amount of an acid catalyst. The acid catalyst is said to contain boron trifluoride in the form of hydroxy- and alkoxy-fluoboric acid, potentially with complexed alcohols. The second step is the alcoholysis of the intermediate complex formed in the first step to produce the desired ester.
A process suffering from a highly corrosive operating environment and requiring a large excess of wet hydrofluoric acid (as the catalyst) is disclosed in GB 1,167,116 assigned to Shell Internationale Research Maatschappij N.V. The process involves the one-step synthesis of carboxylic acids by reacting a branched C.sub.4-10 olefin with carbon monoxide in the presence of a large excess of hydrofluoric acid (preferably a hydrogen fluoride/olefin ratio of about 10:1) and about a 50% molar excess of water. As with a number of processes discussed above, this process has a serious disadvantage in that the hydrofluoric acid must be separated from a product mixture which contains excess water. Hydrofluoric acid is also extremely corrosive and toxic.
A process similar to that disclosed in GB 1,167,116 is shown in GB 1,174,209 also assigned to Shell. GB '209 also discloses steps for recovery and recycle of at least a portion of the hydrofluoric acid catalyst.
Another process using an acid catalyst in an amount more than equimolar to the olefin feed is found in U.S. Pat. No. 3,527,779 to Paulis et al. That process uses a boron trifluoride-water phosphoric acid catalyst which is said to be less corrosive than hydrofluoric acid or boron trifluoride hydrate and more readily recovered from the carboxylic acid product. The catalyst is, however, used in molar excess to the feed olefin. It may be recovered in a subsequent step for recycle.
Much of the literature discussed here is limited to the reaction of higher olefins to produce saturated carboxylic acids. In U.S. Pat. No. 4,256,913 to Jung et al. the disclosed process is limited to the carbonylation of lower non-branched olefins, ethylene and propylene, to the corresponding carboxylic acids. A description for the recovery and reuse of the catalyst is not shown.
The U.S. Pat. No. 4,311,851 to Jung et al. discloses a process for producing carboxylic acid esters and for recovering and reconstituting the boron trifluoride-alcohol catalyst. This process involves a very complicated series of steps for recovery of the catalyst: the operation is carried out until one-half of the alcohol feed is consumed, any remaining free BF.sub.3 is vaporized, additional alcohol is added to the reaction medium, and the reaction mixture is distilled. An azeotrope of the alcohol and the product ester is the distillation product. Heavy by-products are removed from the distillation bottoms by solvent extraction. The treated bottoms are mixed with added BF.sub.3 to form the original catalyst mixture. A simpler process would be desirable.
The U.S. patent to Gelbein (U.S. Pat. No. 4,262,138) is a variation of the process shown in Jung et al. '851 (discussed above) but also appears to be specific to the carbonylation of ethylene and propylene.
A published European Patent Application (No. 0,249,976) assigned to BASF AG discloses a process for the production of carboxylic acids from the reaction of an olefin with carbon monoxide and water over a zeolite catalyst or a modified zeolite catalyst. This process uses a catalyst which is easy to handle and has low corrosivity. However, the process exemplifies high yields of carboxylic acids only at high temperatures and pressures.
Our process is one which uses solid acid catalysts optionally in combination with minor amounts of adjunct Lewis acids. None of the materials cited above utilize their "catalysts" in truly catalytic amounts in producing trialkylacetic acids.